1. Field of the Invention
Generally the field of this invention encompasses fiberglass reinforcement composed of the glass of this invention, which is a relatively low modulus of elasticity glass for new fiberglass applications we believe to be heretofore unknown.
One of the primary areas of utility for the material of this invention being to present a flexible, secondary high energy absorption mass for body armor.
Another application for this type material as utilized in a resin laminate may be for the manufacture of springs. Modulus of resilience is inversely proportional to elastic modulus, hence the lower the elastic modulus, the higher the resilience modulus, permitting a much softer response and lower frequency response to deflection than heretofore possible with thermosetting, fiberglass reinforced laminates reinforced with, for example, conventional "E" glass having a relatively high elastic modulus on the order of 10.5 .times. 10.sup.6 psi.
Another area of possible use of this glass is for automotive tire cords.
2. Description of the Prior Art
The prior art is replete with thousands of patent publications dealing with the fiberglass art generally, wherein various thermosetting plastic resins, such as epoxy and polyester, are formed into such shapes as liquid storage tanks, boat hulls, various and sundry machinery and appliance parts, in combination with fiberglass reinforcement, usually of well-known "E" glass in the form of chopped strand, chopped strand mat, woven roving, or woven strand, to mention a few, whereby the glass filaments lend "muscle" and rigidity to the fiberglass reinforced article.
High modulus glasses are normally used to impart the maximum degree of strength and rigidity to the laminate to adapt it to the various rigorous and demanding stresses it would normally be subjected to.
Consequently, little thought has been given to deliberately investigating the area of low modulus glasses because springyness, resilience and low modulus were not only undesirable for normal uses, there were many reasons why it was felt that a low modulus glass would simply not lend itself to being drawn into filaments, an essential quality if a glass is going to be used as a reinforcing agent in a plastic laminate.
For one thing, the glasses of this invention, as is true of any fiberizable glass, should have relative freedom from crystallization in or near the vicinity of the drawing range, since crystallization occurring during drawing tends to break filaments and make for discontinuity in the strand manufacture. Quite surprisingly, we have found that the preferred glasses of this invention develop literally no liquidus temperature (threshold of crystallization) utilizing a standard test for a normal length of time.
For example, the following test is utilized for determining the liquidus, or crystallization temperature.
A platinum boat, approximately 5 inches long, is filled with a pulverized sample of the glass to be tested, the boat is placed in a conventional recording, temperature gradient furnace and allowed to stabilize substantially at the gradient ambient of the furnace for about a minimum of 15 hours following which the boat is rapidly removed from this furnace and permitted to quickly "freeze," in a matter of seconds at room temperature thereby quickly fixing the glass, so to speak, at whatever crystalline or amorphous state existed in each incremental temperature gradient section of the sample while in the furnace.
When so tested, standard "E" glass will crystallize in a matter of minutes. However, any of the preferred glasses of this invention when so tested, and left at any temperature for at least 48 hours, showed no sign of crystallization.
Another reason why low modulus glasses have never been seriously considered for commercial applications is that the normal expectation is that a low modulus glass will also have a correspondingly low tensile strength, so low, in fact, that it lacks sufficient strength to be drawn from a bushing and fiberized.
However, the low modulus formulations of the glasses of this invention, as will be noted hereinafter in the disclosure, retain a surprisingly high degree of tensile strength permitting them to be readily and quickly drawn from commercial bushings.
Finally, the viscosity curve of the glasses of this invention, as exemplified by the curve of FIG. 1, is much flatter than the viscosity curve of "E" glass for example which also makes for much more trouble-free drawing from a commercial bushing.
This flat viscosity curve allows for a wide temperature range through which fibers can be continuously drawn. A single glass composition of this invention has been found to have a drawing temperature range of over 700.degree. F. e.g. in the attached Table II glass No. 5 was drawn at temperatures from about 1800.degree. to about 2500.degree. F.